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The formation of porous GaAs in HF solutions
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Applied Surface Science 119 (1997) 160-168
The formation of porous GaAs in HF solutions G. Oskam a,,, A. Natarajan a, P.C. Searson a, F.M. Ross b a Department of Materials Science and Engineering, The Johns Hopkins University, 34th and Charles Street, Baltimore, MD 21218, USA b National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 6 February 1997; accepted 2 March 1997
Abstract
The electrochemical etching of n-type GaAs in HF solutions in the dark results in the formation of a porous layer. The pore density, the pore dimensions and the structure of the porous layer depend on the doping density and the crystallographic orientation of the surface. The pore morphology of porous GaAs is essentially independent of the applied current. The pore front velocity is linearly proportional to the current and the porous layer can be grown to any thickness. The primary pores in GaAs grow in the (111 ) a direction which is in contrast with silicon where the pores grow in the (100) direction. The pore diameters increase from 80 nm for highly doped GaAs to 400 nm for undoped GaAs. The combination of electrochemical methods and structural analysis techniques, including transmission electron microscopy and small angle neutron scattering, leads to a better understanding of anisotropic etching of semiconductors. © 1997 Elsevier Science B.V. PACS. 73.40.Mr; 81.60.Cp; 82.45.+ z; 61.16.Bg Keywords: Porous GaAs; Electrochemical pore formation; TEM; SANS
1. Introduction
Isotropic and anisotropic wet etching of semiconductors are widely used in device fabrication [1-5]. An extreme case of anisotropic etching is the formation of porous layers. The unique electrical and optical properties of porous silicon have been used in the fabrication of a wide range of novel structures [5-9]. Silicon-on-insulator (SO1) structures have been fabricated by thermal oxidation of a buried porous silicon layer created by etching in HF solution [6] and devices using full isolation by oxidized porous silicon (FIPOS) on n - / n + / n - wafers have been
* Corresponding author. Tel.: + 1-410-5166452; fax: + 1-4105165293; e-mail: [email protected].
reported [7,8]. Recently, it has been shown that light emitting devices based on porous silicon can be integrated into microelectronic circuits [9]. In general, porous structures can be formed if the etch rate is dependent on the crystallographic orientation, the transport of reactants ( a n d / o r products) is not rate limiting, and the etching process does not lead to the formation of a passivation layer whose dissolution is rate limiting. These considerations illustrate that controlled anisotropic etching can be obtained in semiconductors other than silicon provided that the conditions described above are satisfied [10-14]. Krumme and Straumanis [13] and Faktor and co-workers [14] reported on the electrochemical etching of n-type GaAs in the dark in KOH and H2SO 4 solutions. They found that a porous layer
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII SO 169-43 3 2(9 7 ) 0 0 1 8 4 - 0
G. Oskam et al. / Applied Surface Science 119 (1997) 160-168
was formed with 'tunnels' of diameters ranging from 0.5 /~m to 5 /.tm and a pore density of about 108 cm -2. Further, the tunnels were reported to propagate only in the <111) direction from gallium to arsenic, denoted as the (111)a direction [15]. In this paper we present results on pore formation in n-type GaAs in HF solutions in the dark. We focus on the structural characterization of the porous layers and on the parameters which determine the structure. Results from electrochemical measurements, transmission electron microscopy (TEM) and small angle neutron scattering (SANS) are presented and, based on the results, mechanisms for pore formation are discussed.
2. Experimental Porous layers were grown in single crystal GaAs wafers. The n-type GaAs samples (Laser Diode Products, Inc.) with a (100) orientation and free carrier densities of 2 × 1016 cm --3 and 4 × 1018 cm -3 are denoted as n - GaAs (100) and n ÷ GaAs (100), respectively. The n-type GaAs (111) samples (MCP Electronic Materials, Ltd.) had a doping level of 4 × 1017 cm -3, which is intermediate between the n - GaAs (100) and n ÷ GaAs (100) samples. The dopant was silicon except for the n - GaAs (100) which was undoped. The n-GaAs (111) wafers were always etched from the gallium terminated surface. Porous GaAs layers for transmission electron microscopy (TEM) and small angle neutron scattering (SANS) were prepared by etching at a constant current in 25 M HF (49 wt%) in the dark. The porous layers were grown by clamping a GaAs wafer between the compartments of a two compartment cell using Pt mesh as working electrode and counter electrode. The solution was stirred in all experiments. For the electrochemical measurements, an ohmic contact was made to the back side of the GaAs wafer by electroless deposition of a thin gold layer followed by application of I n - G a eutectic. The contact was annealed for 30 min at 400°C. The experiments were performed in a stirred 25 M HF solution in the dark with a platinum mesh as counter electrode and an HF-resistant saturated calomel electrode (SCE) as reference. A Solartron ECI 1286 potentiostat and a Solartron 1255 Frequency Re-
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sponse Analyzer were used for the electrochemical experiments. Prior to all experiments, wafers were dipped in concentrated HCI (3 min) to remove the native oxide layer and were etched in 1 vol% Br2/methanol (30 s). All porous layers were grown to approximately 20 /~m in thickness. Cross-sectional TEM samples were prepared by grinding, polishing and ion milling, with final milling carried out at liquid nitrogen temperature using 4 kV argon ions at an angle of incidence of 14°. The porous layers were also examined by scraping off fragments onto a holey carbon support [16]. Observations were made using the 800 kV Berkeley Atomic Resolution Microscope and a JEOL 200 CX operated at 200 kV. The SANS experiments were carried out on the 8 m beam line at NIST (Gaithersburg, MD). Samples were mounted so that the neutron beam was perpendicular to the sample surface. The wavelength was 5 A and the distance between the samples and the detector (0.64 × 0.64 m 2) was 5.0 m. Scattering data were collected for about 2 h on each sample.
3. Results and discussion 3.1. Pore morphology
Transmission electron microscopy (TEM) was used to study the structure of the porous layers prepared at various current densities on samples with different substrate orientations and doping levels. The structure of the porous layer was found to depend mainly on the doping density of the substrate and, therefore, the results will be discussed in the order from high to low doping density. The uniformity of the porous layer was found to decrease with decreasing doping density. The layers formed in n ÷ GaAs (100) samples were uniform both laterally and in thickness. The porous layers in the n-GaAs (111) samples were relatively uniform although their thickness fluctuated by as much as 20% on different parts of the wafer. Porous layers formed in n - GaAs (100) were characterized by rectangular surface regions of 200 × 300 /zm where pores were initiated, while other parts of the surface remained unetched. In Fig. 1, TEM images are shown for porous layers formed in n ÷ GaAs (100), n-GaAs (111), and n- GaAs (100) at an etching current density of 10 mA cm -2.
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Fig. la shows a cross sectional image of porous n ÷ GaAs (100) viewed in the [110] zone. It can be seen that pores are formed in four (111) directions with two sets of pores visible in the (110) plane and two sets inclined to the plane. Observation of the pore tips [17] shows that the pores only grow in the
(111 ) directions from gallium to arsenic, denoted as the ( l l I ) a direction [13,15]. From the (100) surface, two ( l l l ) a directions lead into the wafer and the other two go back towards the surface. Analysis of the images reveals that in n ÷ GaAs (100) extensive side branching occurs, leading to pore growth in all four (111)a directions. The morphology of the porous layers is essentially independent of the etching current density, although the pore size increases slightly with increasing current density (Fig. 4). The pore diameter is the same throughout the porous layer, and the GaAs remaining between the pores is crystalline [ 17]. The porous layers formed in n-GaAs (111) and n - GaAs (100) also have pores in the (111)a directions although side branching is less commonly observed. The nucleation of pores was not homogeneous over the surface, particularly for the n- GaAs (100) samples. The morphology of the porous layers in n - GaAs (100) and the average pore diameter were found to be independent of the applied current density (Fig. 4); the current density was not varied for the n-GaAs (111) specimens. Due to the nature of the porous layers formed in both n - GaAs and n-GaAs (111) it was not possible to obtain reliable values for the pore density and the porosity. Fig. l b - d show cross sectional images of the
Fig. 1. Bright field TEM images showing the structure of porous GaAs. The layers were formed at 10 mA cm -2 for 1 h in 25 M HF in the dark. (a) Pores formed in n + GaAs (100) with a donor density of 2X 10 Is cm -3. The specimen is an ion milled cross section imaged in the [110] orientation and the growth direction is from top to bottom. Two set of pores can be seen in the plane of the foil while the other two are inclined. (b) Higher magnification image of a crushed fragment of the same specimen as in (a), viewed down one of the (111) pore propagation directions. The distorted hexagonal shape of the pores is visible. (c) Pores formed in n-type (111) A-face GaAs with a donor density of 4 × 10 t7 cm -3 at the same magnification as (b). The specimen is an ion milled cross section viewed down one of the (111) pore directions. The pores are larger and almost triangular. Note the presence of material within the pores. We do not believe this to be an artifact of specimen preparation since it is also observed in crushed fragments of the same material. (d) Pores formed in n-type GaAs (100) with a donor density of 3 × 1016 c m - 3; an ion milled cross section under the same conditions as (c). The pores are similar to those shown in (c) but are slightly larger. The parallel orientation of the pores is visible
G. Oskam et al. / Applied Surface Science 119 (1997) 160-168
pores viewed down one of the (111) growth directions for all three samples. The pore dimensions were inversely proportional to the donor density: the average pore diameter for the n ÷ GaAs (100) was 80 nm, the pore diameter of the n-GaAs (111) samples was about 220 nm, while the pores in n - GaAs (100) were as large as 400 nm. In addition, the symmetry of the pore cross sections are different; in highly doped n + GaAs (100) the pores were hexagonal in shape, while distorted triangles were observed for the lower doping densities. Fig. lc shows the presence of amorphous material within the pores for n-GaAs (111). This is probably insoluble GaF 3 • 3H20 since the various gallium and arsenic oxides as well as the relevant arsenic salts are soluble in aqueous solutions of low pH [18]. The crystallography of the porous structure has been described in detail elsewhere [ 17]. In Fig. 2, small angle neutron scattering (SANS) contour images are shown for the porous layers prepared in n - GaAs (100), n-GaAs (111) and n + GaAs (100) at an etching current density of 10 mA cm -2 in 25 M HF. The porous layers consist of crystalline GaAs with pores aligned in distinct crystallographic directions. The neutron beam was perpendicular to the surface of the samples and the scattering can be envisaged to take place on the projected image of the pores in the plane of the wafer surface. As a consequence, the preferred directions of the pores, and their relative contributions, can be determined directly by SANS [19]. The scattering images represent the entire porous layers, unlike TEM, which is a more useful technique to determine the microscopic pore shapes and sizes. From Fig. 2 it is clear that the scattering is not isotropic which confirms that the GaAs has etched in preferred crystallographic directions forming a highly anisotropic structure. As described above, analysis of the TEM images revealed that the pores only grow in the (111) directions from gallium to arsenic. The projection of these pores on the (100) plane consists of two sets of rods at an angle of 90°: the two (l l l ) a directions into the wafer and the two directions back towards the surface. As a result, for the (100) oriented surface four preferred directions are expected in the scattering image. The contour plot for the porous layer in n + GaAs (100) shown in Fig. 2a exhibits a square symmetry
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Fig. 2. SANS contour plots of porous GaAs samples oriented perpendicular to the neutron beam (A = 5 A). Scattering data were collected for about 2 h. The plots show intensity contours in a 0.16 × 0.16 A - 2 scattering parameter field. Note that the contour plots give information on the pore orientations; since adequate quantitative models for anisotropic structures are not yet available these data should not be used to determine pore sizes. The porous layers were grown at 10 m A c m -2 for 1 h in 25 M HF in the dark. (a) n + GaAs (100), (b) n-GaAs (111), (c) n - GaAs (1130).
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indicating that all four (111)a directions are equally represented. For the n-GaAs (111) sample, the pores grow preferentially in the three (111)a directions at an angle of 54 degrees to the surface. The projection onto the (111) surface plane corresponds to three sets of rods at an angle of 120°, which would lead to a hexagonal symmetry in the contour plot. As can be seen in Fig. 2b, the hexagonal contour image confirms that the three preferred directions of pore growth are equally represented. As a consequence of the symmetry, the degree of side branching for the (111) samples cannot be established from the SANS contour image. The SANS plot of the n - GaAs (100) sample (Fig. 2c) shows a diamond-like symmetry indicating that two of the four ( l l l ) a directions are preferred, corresponding to the two directions leading into the wafer. The difference in symmetry is correlated to the difference in the degree of side branching, and it can be inferred that side branching results in the formation of pores in the two ( l l l ) a directions which go back towards the surface.
3.2. Electrochemical aspects of pore formation The electrochemical aspects of the pore formation process were studied in order to correlate the fabrication parameters to the resulting structure of the porous layers. Fig. 3 shows the potential of the GaAs electrodes as a function of time during etching in the
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Fig. 3. The measured potential as a function of time during constant current etching in the dark in 25 M I-IF at 10 mA cm -2 for: (a) n + GaAs (100), (b) n-GaAs (111) and (c) n - GaAs (100). In all cases, the potentials reached a steady-state value after 20-30 min.
dark at a constant current of 10 mA cm -2. The n + GaAs (100) samples quickly attain a constant potential of about 0.78 V(SCE); the n-GaAs (111) samples exhibit a constant potential of 2.0 V(SCE) after about 5 min. The n - GaAs (100) samples, however, show a complicated transient with a steady state pore formation voltage of about 8.0 V(SCE). The voltage transients contain information on both the nucleation and the growth of the pores. During the initial stages of pore formation many nuclei may be created at the surface although only a fraction of the nuclei will be able to propagate. The selection process is controlled by the screening effect caused by the characteristic pore spacing which is related to the depletion layer width [5,20]. Once the pore depth reaches several pore spacing lengths, the selection process is complete and stable pore propagation occurs. The constant potential region shows that propagation is controlled by the etching process itself and not by transport of products and/or reactants in the pores [5,20]. Under these conditions stable pore propagation should result in a constant pore size and pore front velocity, and hence pore density and porosity. The spatial inhomogeneity of pore nucleation observed with TEM for the n - GaAs (100) and the n-GaAs (111) samples can also be partially responsible for the complicated transient curves. Since the preferred growth directions are not perpendicular to the sample surface, an additional screening effect may be induced in which pores cannot propagate as they meet pores propagating in another (111 )a direction. An inability to form side branches then leads to the presence of large portions of the sample which cannot be reached by the pores. The Faradaic efficiency of the etching process was determined from weight loss measurements. For the formation of a porous layer in n + GaAs (100) at l0 mA cm -2, 6.0 charges/molecule GaAs were consumed which is in accordance with previously reported values for isotropic etching of GaAs in various solutions [21,22]. The thickness of the porous layer after etching for 1 h was determined by optical microscopy and, using Faraday's law, the porosity was estimated to be about 25%. The pore front velocity during etching at various current densities was determined by preparing porous layers with a thickness of about 20/~m and assuming that the pore front velocity is time independent, as indicated by
G. Oskam et al. / Applied Surface Science 119 (1997) 160-168
the constant potential during stable pore propagation. Fig. 4 shows that the pore front velocity is proportional to the etching current density; the solid line corresponds to a slope of 1. Fig. 4 also illustrates the results described earlier that the pore diameter, determined by TEM, increases only slightly with increasing current density while the morphology and porosity of the layers were found to be essentially independent of the current density. Consequently, increasing the current density mainly increases the pore front velocity and does not significantly affect the pore size and spacing. The small deviation from the slope of 1 at lower currents indicates that the porosity may increase slightly with decreasing etching current. For pore formation in n - GaAs (100) at 10 mA cm-2, weight loss measurements suggested that 7 - l 0 charges are consumed per molecule of GaAs, which indicates that a side reaction takes place. Since gas evolution was observed at the n - GaAs (100) electrodes, the additional charge can be ascribed to the oxidation of water to oxygen. Fig. 5 shows current-potential curves for porous n ÷ GaAs (100) and n - GaAs (100). Reproducible curves were obtained by recording the currentpotential curves on samples with pre-existing porous layers, which were formed in the dark at 10 mA
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u / V(SCE) Fig, 5. Current-potential curves for n + GaAs (100) and n - GaAs (100). The curves were recorded at 10 mV s - i after formation of a porous layer at 10 mA cm -2 for one hour in the dark, The potential sweeps started at 0 V(SCE).
cm -~. This approach minimizes effects due to pore nucleation. In addition, since the pore size is only weakly dependent on the current density, the pore morphology can be considered to be a constant. The curves shown were recorded at a scan rate of 10 mV s -~. For n ÷ GaAs (100) the curves were independent of scan rate from 1-100 mV s-~ whereas for n - GaAs (100) the curves depended considerably on the scan rate. The flat band potentials of both samples, indicated in Fig. 5, were determined prior to pore formation using impedance measurements in the potential range negative of the onset potential for etching. The Mott-Schottky plots and the cathodic part of the current-potential curves have been reported elsewhere [23]. The onset of a large anodic current on n ÷ GaAs (100) is observed at a band bending of only about 1.5 eV while about 3 - 4 eV is needed for n - GaAs (100), indicating that the current is due to a breakdown process and is determined by the electric field at the surface [5,20]. The curve for n - GaAs (100) strongly resembles the currentpotential curves of n - Si in HF solutions in the dark. The current maximum in the curve is generally ascribed to a change in etching mechanism from pore formation to electropolishing [5]. Since the pore diameter is independent of the etching current, it is not likely that electropolishing plays a role in determining the pore size for samples prepared at currents < 10 m A c m - 2
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3.3. Pore propagation The mechanism of pore formation in semiconductors remains largely unknown. In this section we provide evidence that pore propagation in n-type GaAs in the dark is sustained by holes in the valence band generated through a breakdown mechanism. The influence of oxidation intermediates at the surface with energy levels in the bandgap is described qualitatively and we suggest that the etch rate is determined by the capture of holes on surface atoms. Finally, we discuss the influence of the (surface) diffusion length of holes and the electronic configuration of surface atoms on the resulting pore morphology. Although the following discussion is largely qualitative, it provides a self-consistent framework for understanding pore propagation in n-type GaAs in the dark. The electrochemical etching of GaAs requires six charges to dissolve one molecule and, as a consequence, the dissolution reaction is complicated and involves various intermediates of different electronic structure and electrical properties [ 1]. Etching usually proceeds through trapping of valence band holes in surfacebonds, however, the equilibrium density of holes in n-type GaAs in the dark is very low. As a consequence, significant etching can only occur at relatively high voltages where sufficiently large hole densities can be generated by a breakdown mechanism. The anisotropy of the etching reaction further complicates the mechanism since local electric fields may vary on different parts of the surface. Fig. 6 shows a band diagram for a non-porous n-type semiconductor surface at high band bending in the dark. Electrons at the surface can be thermally excited from the valence band to the conduction band although this process is expected to be relatively slow due to the 1.43 eV band gap. This process may be facilitated, however, by the presence of energy levels in the band gap at the surface corresponding to, for instance, oxidation intermediates which have been produced by a chemical oxidation mechanism. Electrons in the conduction band created at the surface can gain sufficient kinetic energy in a large electric field to generate new electron-hole pairs upon impact with an atom thus breaking lattice bonds [24]. This avalanche multiplication process generates free holes which can partici-
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Fig. 6. A band model of an n-type semiconductor under deep depletion conditions. Free holes in the valence band can be generated by tunneling (i) or thermal excitation (not shown) of valence band electrons to the conduction band. Once oxidation intermediates (X) are formed, the etching process can proceed either by (field assisted) thermal excitation (ii) or tunneling (iii) of electrons from X to the conduction band. For a sustained etching, the intermediates have to be regenerated which can occur through trapping of holes on surface atoms (not shown).
pate in the etching process. Alternatively, valence band electrons at the surface can tunnel to the conduction band if a large electric field is present, also creating free holes in the valence band [24]. The capture of free holes by surface sites and the subsequent etching steps create oxidation intermediates at the surface with an energy in the band gap. Further etching can proceed either by the capture of 5 more holes, or by (field assisted) thermal excitation or tunneling of electrons from these surface states into the conduction band. These electrons can again be involved in avalanche multiplication. In order to sustain the etching process, the oxidation intermediates have to be regenerated which can be achieved by the trapping of free holes at surface sites. The potential required for etching at 10 mA cm-2 is a function of the donor density as can be seen from Fig. 3: the higher the donor density, the lower the potential. From the measured potentials and the measured flat band potentials, the band bending during pore formation can be estimated. Using the abrupt junction approximation and assuming that the band edges do not shift significantly during pore formation, the electric fields at the surface during etching for n- GaAs (100) and n + GaAs (100) were determined to be 3 X 105 V cm-~ and 1.5 × 106 V c m -1 , respectively. These values are in good agreement with values calculated for avalanche multiplica-
G. Oskam et aL / Applied Surface Science 119 (1997) 160-168
tion processes on n- GaAs (100), and for tunneling on n + GaAs (100) [24]. Furthermore, the maximum field at the surface increases with increasing donor density which is in accordance with the literature [24]. These observations suggest that avalanche multiplication a n d / o r tunneling are responsible for the hole generation and that free holes in the valence band are available for the etching process. The rate of hole generation is determined by the field at the surface and is, due to the complex three dimensional structure of the porous layer, expected to be spatially dependent. It has been shown previously that due to the small radius of the pore tip, the electric field at the tip can be larger than at the pore walls [12,20,25-27]. As a consequence, the generation rate of holes at the pore tip is higher than at the pore walls. The depletion layer provides a screening effect so that the field at the pore walls becomes even smaller with respect to the field at the tips. The widths of the depletion layers during stable pore propagation (calculated for a flat surface) are about 710 nm and 22 nm for n - GaAs (100) and for n + GaAs (100), respectively. Based on these considerations, quantum sized features are not expected for porous n-GaAs prepared in the dark, in agreement with the experimental results. Similar results have been reported for porous n-Si [27] and porous n-GaP prepared in the dark [28]. The decrease in pore dimensions with increasing donor density can be explained in the following way. Since the rate of hole capture (etching) is generally much slower than the generation rate, the holes at the pore tip can be considered to be in quasi-equilibrium with the bulk. In this case, as the donor density increases, the field at the surface also increases thereby confining the holes closer to the surface. In addition, the hole diffusion length is expected to decrease with increasing donor density [24,29]; the combination of these two effects will lead to localization of the holes closer to the pore tip and hence to decreased pore size. Since the majority of the holes are generated at the pore tips, the holes have to diffuse along the perimeter of the pores over relatively long distances in order to attain equilibrium. At locations on the microscopic surface where the concentration of weakly bonded sites is large, the hole capture probability is larger which leads to a decrease of surface
167
diffusion length of free holes. As the porous layer becomes thicker, pore widening is thus effectively halted by the depletion of holes at the pore walls, which agrees with the observation that the pore diameter is constant over the thickness of the porous layer. The electrochemical etch rate of semiconductors is often limited by the first hole capture at a surface site [30] and this initial step is expected to occur preferentially at weakly coordinated sites. Subsequent reaction steps are expected to be faster than the first step and they should have no influence on the resultant pore structure. Consequently, the etch rate will be larger at the pore tips where the concentration of weakly coordinated surface sites and the density of holes is expected to be higher, which leads to a highly directional etching process resulting in pore propagation.
4. Conclusions The electrochemical etching of n-GaAs in HF solutions in the dark was studied by conventional electrochemical techniques and by structural analysis methods including TEM and SANS. The etching is anisotropic and the pore density, pore dimensions and pore shape are determined by the doping density and the crystallographic orientation of the surface. The primary pores in GaAs grow in the ( l l l ) a directions from gallium to arsenic. In undoped nGaAs (100) the pores mainly run in the two ( l l l ) a directions away from the surface. In highly doped n-GaAs (t00) side branching is extensive and the two ( 111 )a directions back to the surface are equally represented. The combination of the results from electrochemical methods and structural analysis leads to a better insight into the electrochemical pore formation in semiconductors.
Acknowledgements The authors would like to thank D.G. Wiesler (Department of Physics, George Washington University, Washington, DC) for assistance with the neutron scattering experiments and J.M. Macaulay (Silicon Video Corporation, San Jose, CA) and J.A.
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Liddle (Lucent Technologies, Murray Hill, NJ) for helpful discussions. We are also grateful to Professor J.J. Kelly (Universiteit Utrecht, The Netherlands) for donating the n-GaAs (111) material. This work was partially supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division, U.S. Department of Energy under contract No. DE AC-03-76SF00098, and by the National Science Foundation under grant DMR9202645. References [1] P.H.L. Notten, J.E.A.M. van den Meerakker, J.J. Kelly, Etching of III-V Semiconductors: An Electrochemical Approach, Elsevier, Oxford, 1991. [2] Y. Tarui, Y. Komiya, Y. Harada, J. Etectrochem. Soc. 118 (1971) 117+ [3] D.V. Podlesnik, H,H. Gilgen, R.M. Osgood, Appl. Phys. Lett. 45 (1984) 563. [4] S. Ottow, V. Lehmann, H. F6II, J. Electrochem. Soc. 143 (1996) 385. [5] R.L. Smith, S.D. Collins, J. Appl. Phys. 71 (8) (1992) RI. [6] X.-Z. Tu, J. Vac. Sci, Technot. B 6 (1988) 1530, [7] Y. Watanabe, Y. Arita, T. Yokoyama, Y. Igarashi, J. Electrochem. So(=. 122 (1975) t351. [8] S.S. Tao, D.R. Myers, T.R. Guilinger, M.J. Kelly, J. Appl. Phys. 62 (1987) 4182. [9] K.D. Hirschman, L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Nature 384 (1996) 338. [10] H.C. Gatos, M.C. Lavine, J. Etectrochem. Soc. 107 (1960) 433.
[t 1] B.D. Chase, D.B. Holt, J. Electrochem. Soc. 119 (t972) 314. [12] M.I.J. Beale, J.D. Benjamin, M.J. Uren, N.G. Chew, A.G. Cullis, J. Cryst. Growth 73 (1985) 622. [ 13] J.-P. Krumme, M.E. Straumanis, Trans. Met. Soc. AIME 239 (1967) 396. [14] M.M. Faktor, D.G. Fiddyment, M.R. Taylor, J. Electrochem. Soc. 122 (1975) 1566. [t5] D.B. Holt, J. Mater. Sci. 23 (1988) 1131. [16] L.T. Canham, A.G. Cullis, Nature 353 (1991) 335. [17] F.M. Ross, G. Oskam, P.C. Searson, J,M. Macaulay, J,A. Liddle, Phil. Mag. A, 1997, in press. [i 8] Gmelin's Handbuch der Anorganische Chemie, 7th ed., Vol. 36, Winter, Heidelberg, 1932, p. 72. [19] G.E. Bacon, Neutron Diffraction, 3rd ed., Clarendon Press, Oxford, 1975. [20] P.C. Searson, J.M. Macaulay, F.M. Ross, J. Appl. Phys. 72 (1992) 253. [21] H. Gerischer, I. Mattes, Z. Phys. Chem. 49 (1966) 112. [22] D. Vanmaekelbergh, W.P. Gomes, J. Phys. Chem. 94 (1990) 1571. [23] G. Oskam, A. Natarajan, P.C. Searson, F.M. Ross, J.M. Macaulay, J.A+ Liddle, in: Proc. of the State-of-the-Art Program on Compound Semiconductors XXI, Electrochemical Society, Pennington, NJ, 1995, p. 147. [24] S.M. Sze, Semiconductor Devices: Physics and Technology, John Wiley and Sons, New York, 1985, pp. 100-107. [25] V. Lehmann, H. F~I1, J. Electrochem. Soc. 137 (1990) 653. [26] X.G. Zhang, S.D. Collins, R.L. Smith, J. Electrochem. Soc. 136 (1989) 1561. [27] P.C. Searson, J.M. Macaulay, S.M. Prokes, J. Electrochem. Soc. 139 (1992) 3373. [28] B.H. Ern~, D. Vanmaekeibergh, J.J. Kelly, Adv. Mater. 7 (1995) 739. [29] D. Soltz, L. Cescato, J. Electrochem. Soc. 143 (1996) 28t5. [30] H. Gerischer, Bet. Bunsenges. Phys. Chem. 69 (1965) 578.
surface science ELSEVIER
Applied Surface Science 119 (1997) 160-168
The formation of porous GaAs in HF solutions G. Oskam a,,, A. Natarajan a, P.C. Searson a, F.M. Ross b a Department of Materials Science and Engineering, The Johns Hopkins University, 34th and Charles Street, Baltimore, MD 21218, USA b National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 6 February 1997; accepted 2 March 1997
Abstract
The electrochemical etching of n-type GaAs in HF solutions in the dark results in the formation of a porous layer. The pore density, the pore dimensions and the structure of the porous layer depend on the doping density and the crystallographic orientation of the surface. The pore morphology of porous GaAs is essentially independent of the applied current. The pore front velocity is linearly proportional to the current and the porous layer can be grown to any thickness. The primary pores in GaAs grow in the (111 ) a direction which is in contrast with silicon where the pores grow in the (100) direction. The pore diameters increase from 80 nm for highly doped GaAs to 400 nm for undoped GaAs. The combination of electrochemical methods and structural analysis techniques, including transmission electron microscopy and small angle neutron scattering, leads to a better understanding of anisotropic etching of semiconductors. © 1997 Elsevier Science B.V. PACS. 73.40.Mr; 81.60.Cp; 82.45.+ z; 61.16.Bg Keywords: Porous GaAs; Electrochemical pore formation; TEM; SANS
1. Introduction
Isotropic and anisotropic wet etching of semiconductors are widely used in device fabrication [1-5]. An extreme case of anisotropic etching is the formation of porous layers. The unique electrical and optical properties of porous silicon have been used in the fabrication of a wide range of novel structures [5-9]. Silicon-on-insulator (SO1) structures have been fabricated by thermal oxidation of a buried porous silicon layer created by etching in HF solution [6] and devices using full isolation by oxidized porous silicon (FIPOS) on n - / n + / n - wafers have been
* Corresponding author. Tel.: + 1-410-5166452; fax: + 1-4105165293; e-mail: [email protected].
reported [7,8]. Recently, it has been shown that light emitting devices based on porous silicon can be integrated into microelectronic circuits [9]. In general, porous structures can be formed if the etch rate is dependent on the crystallographic orientation, the transport of reactants ( a n d / o r products) is not rate limiting, and the etching process does not lead to the formation of a passivation layer whose dissolution is rate limiting. These considerations illustrate that controlled anisotropic etching can be obtained in semiconductors other than silicon provided that the conditions described above are satisfied [10-14]. Krumme and Straumanis [13] and Faktor and co-workers [14] reported on the electrochemical etching of n-type GaAs in the dark in KOH and H2SO 4 solutions. They found that a porous layer
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G. Oskam et al. / Applied Surface Science 119 (1997) 160-168
was formed with 'tunnels' of diameters ranging from 0.5 /~m to 5 /.tm and a pore density of about 108 cm -2. Further, the tunnels were reported to propagate only in the <111) direction from gallium to arsenic, denoted as the (111)a direction [15]. In this paper we present results on pore formation in n-type GaAs in HF solutions in the dark. We focus on the structural characterization of the porous layers and on the parameters which determine the structure. Results from electrochemical measurements, transmission electron microscopy (TEM) and small angle neutron scattering (SANS) are presented and, based on the results, mechanisms for pore formation are discussed.
2. Experimental Porous layers were grown in single crystal GaAs wafers. The n-type GaAs samples (Laser Diode Products, Inc.) with a (100) orientation and free carrier densities of 2 × 1016 cm --3 and 4 × 1018 cm -3 are denoted as n - GaAs (100) and n ÷ GaAs (100), respectively. The n-type GaAs (111) samples (MCP Electronic Materials, Ltd.) had a doping level of 4 × 1017 cm -3, which is intermediate between the n - GaAs (100) and n ÷ GaAs (100) samples. The dopant was silicon except for the n - GaAs (100) which was undoped. The n-GaAs (111) wafers were always etched from the gallium terminated surface. Porous GaAs layers for transmission electron microscopy (TEM) and small angle neutron scattering (SANS) were prepared by etching at a constant current in 25 M HF (49 wt%) in the dark. The porous layers were grown by clamping a GaAs wafer between the compartments of a two compartment cell using Pt mesh as working electrode and counter electrode. The solution was stirred in all experiments. For the electrochemical measurements, an ohmic contact was made to the back side of the GaAs wafer by electroless deposition of a thin gold layer followed by application of I n - G a eutectic. The contact was annealed for 30 min at 400°C. The experiments were performed in a stirred 25 M HF solution in the dark with a platinum mesh as counter electrode and an HF-resistant saturated calomel electrode (SCE) as reference. A Solartron ECI 1286 potentiostat and a Solartron 1255 Frequency Re-
161
sponse Analyzer were used for the electrochemical experiments. Prior to all experiments, wafers were dipped in concentrated HCI (3 min) to remove the native oxide layer and were etched in 1 vol% Br2/methanol (30 s). All porous layers were grown to approximately 20 /~m in thickness. Cross-sectional TEM samples were prepared by grinding, polishing and ion milling, with final milling carried out at liquid nitrogen temperature using 4 kV argon ions at an angle of incidence of 14°. The porous layers were also examined by scraping off fragments onto a holey carbon support [16]. Observations were made using the 800 kV Berkeley Atomic Resolution Microscope and a JEOL 200 CX operated at 200 kV. The SANS experiments were carried out on the 8 m beam line at NIST (Gaithersburg, MD). Samples were mounted so that the neutron beam was perpendicular to the sample surface. The wavelength was 5 A and the distance between the samples and the detector (0.64 × 0.64 m 2) was 5.0 m. Scattering data were collected for about 2 h on each sample.
3. Results and discussion 3.1. Pore morphology
Transmission electron microscopy (TEM) was used to study the structure of the porous layers prepared at various current densities on samples with different substrate orientations and doping levels. The structure of the porous layer was found to depend mainly on the doping density of the substrate and, therefore, the results will be discussed in the order from high to low doping density. The uniformity of the porous layer was found to decrease with decreasing doping density. The layers formed in n ÷ GaAs (100) samples were uniform both laterally and in thickness. The porous layers in the n-GaAs (111) samples were relatively uniform although their thickness fluctuated by as much as 20% on different parts of the wafer. Porous layers formed in n - GaAs (100) were characterized by rectangular surface regions of 200 × 300 /zm where pores were initiated, while other parts of the surface remained unetched. In Fig. 1, TEM images are shown for porous layers formed in n ÷ GaAs (100), n-GaAs (111), and n- GaAs (100) at an etching current density of 10 mA cm -2.
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Fig. la shows a cross sectional image of porous n ÷ GaAs (100) viewed in the [110] zone. It can be seen that pores are formed in four (111) directions with two sets of pores visible in the (110) plane and two sets inclined to the plane. Observation of the pore tips [17] shows that the pores only grow in the
(111 ) directions from gallium to arsenic, denoted as the ( l l I ) a direction [13,15]. From the (100) surface, two ( l l l ) a directions lead into the wafer and the other two go back towards the surface. Analysis of the images reveals that in n ÷ GaAs (100) extensive side branching occurs, leading to pore growth in all four (111)a directions. The morphology of the porous layers is essentially independent of the etching current density, although the pore size increases slightly with increasing current density (Fig. 4). The pore diameter is the same throughout the porous layer, and the GaAs remaining between the pores is crystalline [ 17]. The porous layers formed in n-GaAs (111) and n - GaAs (100) also have pores in the (111)a directions although side branching is less commonly observed. The nucleation of pores was not homogeneous over the surface, particularly for the n- GaAs (100) samples. The morphology of the porous layers in n - GaAs (100) and the average pore diameter were found to be independent of the applied current density (Fig. 4); the current density was not varied for the n-GaAs (111) specimens. Due to the nature of the porous layers formed in both n - GaAs and n-GaAs (111) it was not possible to obtain reliable values for the pore density and the porosity. Fig. l b - d show cross sectional images of the
Fig. 1. Bright field TEM images showing the structure of porous GaAs. The layers were formed at 10 mA cm -2 for 1 h in 25 M HF in the dark. (a) Pores formed in n + GaAs (100) with a donor density of 2X 10 Is cm -3. The specimen is an ion milled cross section imaged in the [110] orientation and the growth direction is from top to bottom. Two set of pores can be seen in the plane of the foil while the other two are inclined. (b) Higher magnification image of a crushed fragment of the same specimen as in (a), viewed down one of the (111) pore propagation directions. The distorted hexagonal shape of the pores is visible. (c) Pores formed in n-type (111) A-face GaAs with a donor density of 4 × 10 t7 cm -3 at the same magnification as (b). The specimen is an ion milled cross section viewed down one of the (111) pore directions. The pores are larger and almost triangular. Note the presence of material within the pores. We do not believe this to be an artifact of specimen preparation since it is also observed in crushed fragments of the same material. (d) Pores formed in n-type GaAs (100) with a donor density of 3 × 1016 c m - 3; an ion milled cross section under the same conditions as (c). The pores are similar to those shown in (c) but are slightly larger. The parallel orientation of the pores is visible
G. Oskam et al. / Applied Surface Science 119 (1997) 160-168
pores viewed down one of the (111) growth directions for all three samples. The pore dimensions were inversely proportional to the donor density: the average pore diameter for the n ÷ GaAs (100) was 80 nm, the pore diameter of the n-GaAs (111) samples was about 220 nm, while the pores in n - GaAs (100) were as large as 400 nm. In addition, the symmetry of the pore cross sections are different; in highly doped n + GaAs (100) the pores were hexagonal in shape, while distorted triangles were observed for the lower doping densities. Fig. lc shows the presence of amorphous material within the pores for n-GaAs (111). This is probably insoluble GaF 3 • 3H20 since the various gallium and arsenic oxides as well as the relevant arsenic salts are soluble in aqueous solutions of low pH [18]. The crystallography of the porous structure has been described in detail elsewhere [ 17]. In Fig. 2, small angle neutron scattering (SANS) contour images are shown for the porous layers prepared in n - GaAs (100), n-GaAs (111) and n + GaAs (100) at an etching current density of 10 mA cm -2 in 25 M HF. The porous layers consist of crystalline GaAs with pores aligned in distinct crystallographic directions. The neutron beam was perpendicular to the surface of the samples and the scattering can be envisaged to take place on the projected image of the pores in the plane of the wafer surface. As a consequence, the preferred directions of the pores, and their relative contributions, can be determined directly by SANS [19]. The scattering images represent the entire porous layers, unlike TEM, which is a more useful technique to determine the microscopic pore shapes and sizes. From Fig. 2 it is clear that the scattering is not isotropic which confirms that the GaAs has etched in preferred crystallographic directions forming a highly anisotropic structure. As described above, analysis of the TEM images revealed that the pores only grow in the (111) directions from gallium to arsenic. The projection of these pores on the (100) plane consists of two sets of rods at an angle of 90°: the two (l l l ) a directions into the wafer and the two directions back towards the surface. As a result, for the (100) oriented surface four preferred directions are expected in the scattering image. The contour plot for the porous layer in n + GaAs (100) shown in Fig. 2a exhibits a square symmetry
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indicating that all four (111)a directions are equally represented. For the n-GaAs (111) sample, the pores grow preferentially in the three (111)a directions at an angle of 54 degrees to the surface. The projection onto the (111) surface plane corresponds to three sets of rods at an angle of 120°, which would lead to a hexagonal symmetry in the contour plot. As can be seen in Fig. 2b, the hexagonal contour image confirms that the three preferred directions of pore growth are equally represented. As a consequence of the symmetry, the degree of side branching for the (111) samples cannot be established from the SANS contour image. The SANS plot of the n - GaAs (100) sample (Fig. 2c) shows a diamond-like symmetry indicating that two of the four ( l l l ) a directions are preferred, corresponding to the two directions leading into the wafer. The difference in symmetry is correlated to the difference in the degree of side branching, and it can be inferred that side branching results in the formation of pores in the two ( l l l ) a directions which go back towards the surface.
3.2. Electrochemical aspects of pore formation The electrochemical aspects of the pore formation process were studied in order to correlate the fabrication parameters to the resulting structure of the porous layers. Fig. 3 shows the potential of the GaAs electrodes as a function of time during etching in the
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dark at a constant current of 10 mA cm -2. The n + GaAs (100) samples quickly attain a constant potential of about 0.78 V(SCE); the n-GaAs (111) samples exhibit a constant potential of 2.0 V(SCE) after about 5 min. The n - GaAs (100) samples, however, show a complicated transient with a steady state pore formation voltage of about 8.0 V(SCE). The voltage transients contain information on both the nucleation and the growth of the pores. During the initial stages of pore formation many nuclei may be created at the surface although only a fraction of the nuclei will be able to propagate. The selection process is controlled by the screening effect caused by the characteristic pore spacing which is related to the depletion layer width [5,20]. Once the pore depth reaches several pore spacing lengths, the selection process is complete and stable pore propagation occurs. The constant potential region shows that propagation is controlled by the etching process itself and not by transport of products and/or reactants in the pores [5,20]. Under these conditions stable pore propagation should result in a constant pore size and pore front velocity, and hence pore density and porosity. The spatial inhomogeneity of pore nucleation observed with TEM for the n - GaAs (100) and the n-GaAs (111) samples can also be partially responsible for the complicated transient curves. Since the preferred growth directions are not perpendicular to the sample surface, an additional screening effect may be induced in which pores cannot propagate as they meet pores propagating in another (111 )a direction. An inability to form side branches then leads to the presence of large portions of the sample which cannot be reached by the pores. The Faradaic efficiency of the etching process was determined from weight loss measurements. For the formation of a porous layer in n + GaAs (100) at l0 mA cm -2, 6.0 charges/molecule GaAs were consumed which is in accordance with previously reported values for isotropic etching of GaAs in various solutions [21,22]. The thickness of the porous layer after etching for 1 h was determined by optical microscopy and, using Faraday's law, the porosity was estimated to be about 25%. The pore front velocity during etching at various current densities was determined by preparing porous layers with a thickness of about 20/~m and assuming that the pore front velocity is time independent, as indicated by
G. Oskam et al. / Applied Surface Science 119 (1997) 160-168
the constant potential during stable pore propagation. Fig. 4 shows that the pore front velocity is proportional to the etching current density; the solid line corresponds to a slope of 1. Fig. 4 also illustrates the results described earlier that the pore diameter, determined by TEM, increases only slightly with increasing current density while the morphology and porosity of the layers were found to be essentially independent of the current density. Consequently, increasing the current density mainly increases the pore front velocity and does not significantly affect the pore size and spacing. The small deviation from the slope of 1 at lower currents indicates that the porosity may increase slightly with decreasing etching current. For pore formation in n - GaAs (100) at 10 mA cm-2, weight loss measurements suggested that 7 - l 0 charges are consumed per molecule of GaAs, which indicates that a side reaction takes place. Since gas evolution was observed at the n - GaAs (100) electrodes, the additional charge can be ascribed to the oxidation of water to oxygen. Fig. 5 shows current-potential curves for porous n ÷ GaAs (100) and n - GaAs (100). Reproducible curves were obtained by recording the currentpotential curves on samples with pre-existing porous layers, which were formed in the dark at 10 mA
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cm -~. This approach minimizes effects due to pore nucleation. In addition, since the pore size is only weakly dependent on the current density, the pore morphology can be considered to be a constant. The curves shown were recorded at a scan rate of 10 mV s -~. For n ÷ GaAs (100) the curves were independent of scan rate from 1-100 mV s-~ whereas for n - GaAs (100) the curves depended considerably on the scan rate. The flat band potentials of both samples, indicated in Fig. 5, were determined prior to pore formation using impedance measurements in the potential range negative of the onset potential for etching. The Mott-Schottky plots and the cathodic part of the current-potential curves have been reported elsewhere [23]. The onset of a large anodic current on n ÷ GaAs (100) is observed at a band bending of only about 1.5 eV while about 3 - 4 eV is needed for n - GaAs (100), indicating that the current is due to a breakdown process and is determined by the electric field at the surface [5,20]. The curve for n - GaAs (100) strongly resembles the currentpotential curves of n - Si in HF solutions in the dark. The current maximum in the curve is generally ascribed to a change in etching mechanism from pore formation to electropolishing [5]. Since the pore diameter is independent of the etching current, it is not likely that electropolishing plays a role in determining the pore size for samples prepared at currents < 10 m A c m - 2
166
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3.3. Pore propagation The mechanism of pore formation in semiconductors remains largely unknown. In this section we provide evidence that pore propagation in n-type GaAs in the dark is sustained by holes in the valence band generated through a breakdown mechanism. The influence of oxidation intermediates at the surface with energy levels in the bandgap is described qualitatively and we suggest that the etch rate is determined by the capture of holes on surface atoms. Finally, we discuss the influence of the (surface) diffusion length of holes and the electronic configuration of surface atoms on the resulting pore morphology. Although the following discussion is largely qualitative, it provides a self-consistent framework for understanding pore propagation in n-type GaAs in the dark. The electrochemical etching of GaAs requires six charges to dissolve one molecule and, as a consequence, the dissolution reaction is complicated and involves various intermediates of different electronic structure and electrical properties [ 1]. Etching usually proceeds through trapping of valence band holes in surfacebonds, however, the equilibrium density of holes in n-type GaAs in the dark is very low. As a consequence, significant etching can only occur at relatively high voltages where sufficiently large hole densities can be generated by a breakdown mechanism. The anisotropy of the etching reaction further complicates the mechanism since local electric fields may vary on different parts of the surface. Fig. 6 shows a band diagram for a non-porous n-type semiconductor surface at high band bending in the dark. Electrons at the surface can be thermally excited from the valence band to the conduction band although this process is expected to be relatively slow due to the 1.43 eV band gap. This process may be facilitated, however, by the presence of energy levels in the band gap at the surface corresponding to, for instance, oxidation intermediates which have been produced by a chemical oxidation mechanism. Electrons in the conduction band created at the surface can gain sufficient kinetic energy in a large electric field to generate new electron-hole pairs upon impact with an atom thus breaking lattice bonds [24]. This avalanche multiplication process generates free holes which can partici-
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Fig. 6. A band model of an n-type semiconductor under deep depletion conditions. Free holes in the valence band can be generated by tunneling (i) or thermal excitation (not shown) of valence band electrons to the conduction band. Once oxidation intermediates (X) are formed, the etching process can proceed either by (field assisted) thermal excitation (ii) or tunneling (iii) of electrons from X to the conduction band. For a sustained etching, the intermediates have to be regenerated which can occur through trapping of holes on surface atoms (not shown).
pate in the etching process. Alternatively, valence band electrons at the surface can tunnel to the conduction band if a large electric field is present, also creating free holes in the valence band [24]. The capture of free holes by surface sites and the subsequent etching steps create oxidation intermediates at the surface with an energy in the band gap. Further etching can proceed either by the capture of 5 more holes, or by (field assisted) thermal excitation or tunneling of electrons from these surface states into the conduction band. These electrons can again be involved in avalanche multiplication. In order to sustain the etching process, the oxidation intermediates have to be regenerated which can be achieved by the trapping of free holes at surface sites. The potential required for etching at 10 mA cm-2 is a function of the donor density as can be seen from Fig. 3: the higher the donor density, the lower the potential. From the measured potentials and the measured flat band potentials, the band bending during pore formation can be estimated. Using the abrupt junction approximation and assuming that the band edges do not shift significantly during pore formation, the electric fields at the surface during etching for n- GaAs (100) and n + GaAs (100) were determined to be 3 X 105 V cm-~ and 1.5 × 106 V c m -1 , respectively. These values are in good agreement with values calculated for avalanche multiplica-
G. Oskam et aL / Applied Surface Science 119 (1997) 160-168
tion processes on n- GaAs (100), and for tunneling on n + GaAs (100) [24]. Furthermore, the maximum field at the surface increases with increasing donor density which is in accordance with the literature [24]. These observations suggest that avalanche multiplication a n d / o r tunneling are responsible for the hole generation and that free holes in the valence band are available for the etching process. The rate of hole generation is determined by the field at the surface and is, due to the complex three dimensional structure of the porous layer, expected to be spatially dependent. It has been shown previously that due to the small radius of the pore tip, the electric field at the tip can be larger than at the pore walls [12,20,25-27]. As a consequence, the generation rate of holes at the pore tip is higher than at the pore walls. The depletion layer provides a screening effect so that the field at the pore walls becomes even smaller with respect to the field at the tips. The widths of the depletion layers during stable pore propagation (calculated for a flat surface) are about 710 nm and 22 nm for n - GaAs (100) and for n + GaAs (100), respectively. Based on these considerations, quantum sized features are not expected for porous n-GaAs prepared in the dark, in agreement with the experimental results. Similar results have been reported for porous n-Si [27] and porous n-GaP prepared in the dark [28]. The decrease in pore dimensions with increasing donor density can be explained in the following way. Since the rate of hole capture (etching) is generally much slower than the generation rate, the holes at the pore tip can be considered to be in quasi-equilibrium with the bulk. In this case, as the donor density increases, the field at the surface also increases thereby confining the holes closer to the surface. In addition, the hole diffusion length is expected to decrease with increasing donor density [24,29]; the combination of these two effects will lead to localization of the holes closer to the pore tip and hence to decreased pore size. Since the majority of the holes are generated at the pore tips, the holes have to diffuse along the perimeter of the pores over relatively long distances in order to attain equilibrium. At locations on the microscopic surface where the concentration of weakly bonded sites is large, the hole capture probability is larger which leads to a decrease of surface
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diffusion length of free holes. As the porous layer becomes thicker, pore widening is thus effectively halted by the depletion of holes at the pore walls, which agrees with the observation that the pore diameter is constant over the thickness of the porous layer. The electrochemical etch rate of semiconductors is often limited by the first hole capture at a surface site [30] and this initial step is expected to occur preferentially at weakly coordinated sites. Subsequent reaction steps are expected to be faster than the first step and they should have no influence on the resultant pore structure. Consequently, the etch rate will be larger at the pore tips where the concentration of weakly coordinated surface sites and the density of holes is expected to be higher, which leads to a highly directional etching process resulting in pore propagation.
4. Conclusions The electrochemical etching of n-GaAs in HF solutions in the dark was studied by conventional electrochemical techniques and by structural analysis methods including TEM and SANS. The etching is anisotropic and the pore density, pore dimensions and pore shape are determined by the doping density and the crystallographic orientation of the surface. The primary pores in GaAs grow in the ( l l l ) a directions from gallium to arsenic. In undoped nGaAs (100) the pores mainly run in the two ( l l l ) a directions away from the surface. In highly doped n-GaAs (t00) side branching is extensive and the two ( 111 )a directions back to the surface are equally represented. The combination of the results from electrochemical methods and structural analysis leads to a better insight into the electrochemical pore formation in semiconductors.
Acknowledgements The authors would like to thank D.G. Wiesler (Department of Physics, George Washington University, Washington, DC) for assistance with the neutron scattering experiments and J.M. Macaulay (Silicon Video Corporation, San Jose, CA) and J.A.
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Liddle (Lucent Technologies, Murray Hill, NJ) for helpful discussions. We are also grateful to Professor J.J. Kelly (Universiteit Utrecht, The Netherlands) for donating the n-GaAs (111) material. This work was partially supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division, U.S. Department of Energy under contract No. DE AC-03-76SF00098, and by the National Science Foundation under grant DMR9202645. References [1] P.H.L. Notten, J.E.A.M. van den Meerakker, J.J. Kelly, Etching of III-V Semiconductors: An Electrochemical Approach, Elsevier, Oxford, 1991. [2] Y. Tarui, Y. Komiya, Y. Harada, J. Etectrochem. Soc. 118 (1971) 117+ [3] D.V. Podlesnik, H,H. Gilgen, R.M. Osgood, Appl. Phys. Lett. 45 (1984) 563. [4] S. Ottow, V. Lehmann, H. F6II, J. Electrochem. Soc. 143 (1996) 385. [5] R.L. Smith, S.D. Collins, J. Appl. Phys. 71 (8) (1992) RI. [6] X.-Z. Tu, J. Vac. Sci, Technot. B 6 (1988) 1530, [7] Y. Watanabe, Y. Arita, T. Yokoyama, Y. Igarashi, J. Electrochem. So(=. 122 (1975) t351. [8] S.S. Tao, D.R. Myers, T.R. Guilinger, M.J. Kelly, J. Appl. Phys. 62 (1987) 4182. [9] K.D. Hirschman, L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Nature 384 (1996) 338. [10] H.C. Gatos, M.C. Lavine, J. Etectrochem. Soc. 107 (1960) 433.
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